1. Field of the Invention
The present application is directed to flight control law architectures.
2. Description of Related Art
Control of aircraft consists of (1) feedback gain design and arrangement, and (2) Logic design of how to switch from one loop to another loop. Method of three loop control laws has provided a fundamental method to change the traditional method (inner loop and outer loop only) for aircraft control gain design to three loop control laws design. The three loop control laws hereby will be extended into the other stage of making all gain sets to be available from vertical takeoff and landing (VTOL) to airplane or from airplane to VTOL flight modes. In addition, the associated logic design using state machines will be collaborated so that the entire flight control computer (FCC) vehicle management system (VMS) architecture will be powerful for manned, unmanned, and optional manned types of flights.
To make the feedback control of an uncertainty system to be robust, stability and control are one of the most important factors. However, these factors will be affected by the control plant model, control sensor measurement feedback, system dead-time delay also known as transport time delay and control command quickness. Several methods have been discussed heavily in the recent years. The first one is the standard model following technology. Model following is indeed one of very powerful methods to improve the control system with better gain margin, phase margin and disturbance rejection. However, the improvement of this method is needed, since it may slow down the pilot's command during the emergency flight characteristics.
The second factor is the dead-time delay on the feedback control computer and sensor feedback measurement. In fact, the entire calculation time-delay in the rotorcraft industries can be resources from the sensor measurement, aerodynamics delay, actuator delay and control input delay from aircraft mechanical link or fly-by-wire (FBW) command. If the sum of the total calculation time delay is too large, the aircraft will become uncontrollable or very hard to control. Therefore, the design of the aircraft plant plays a very important role. Traditionally, the aircraft flight mechanics design must satisfy certain stability requirements such that the aircraft can be handled with and without feedback control laws. Therefore, when aircraft goes into the final degrade mode, the natural flight mechanics mode is used for the pilot command control.
The third method is to apply the inverse plant to help stabilize the aircraft. This method is also very powerful for traditional aircraft control, when the aerodynamics coefficients speed-schedule curves are approximately linear. However, this method may have a problem when the major aerodynamics coefficients cross-over the zero imaginary axis line for new advanced aircraft design. Nowadays, design of the aircraft have moved to multi-operable such as manned, optionally manned and unmanned aircraft, which requires the flight control system to be fly-by-wire type. This type of aircraft can be designed to save the entire gross weight for a more unstable flight mechanics mode. The advantages of this type of aircraft are: (1) Much higher lift coefficient; (2) Shorter take-off range; (3) Shorter control surfaces; (4) Entire aircraft weight saving; and, (5) Smaller engine powers to decrease fuel consumptions.
In addition, for this type of aircraft, the final degrade mode for flight control is not necessary to be the natural flight mechanics mode, because of optional manned and unmanned functions. Depending on availability of sensor redundancy design, the final flight control design mode for this type of aircraft can be either rate command or attitude command/attitude hold or speed hold. Since this type of aircraft will not have the direct link in the aircraft for pilot's maneuver, the command quickness for the manned and unmanned can become challenging tasks for the design engineers.
Recently, new aircraft have been designed with much more unstable flight mechanics modes by taking the above listed benefits. Because of this unstable factor, there may be some major coefficients which cross-over the imaginary axis from stable modes to unstable modes, due to airspeed or C.G. changes. For such a situation, the use of the inverse plant of aircraft model must be designed to avoid dividing by zeroes. Design criteria of aircraft have now moved from a traditional classic feedback method, to a much more challenging stage.
Although the foregoing developments represent great strides in the area of control law systems, many shortcomings remain.
While the system and method of the present application is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular embodiment disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the process of the present application as defined by the appended claims.